Ventilation Is the Movement of Gas into and out of the Lung

The oxygen needs of metabolism require that an animal take a certain volume of air into its lungs, especially its alveoli, each minute. The total volume of air breathed per minute, or minute ventilation (Ve), is determined by the volume of each breath, known as the tidal volume (VT), and the number of

FIGURE 45-2 Diagrammatic representation of the processes involved in gas exchange.

breaths per minute, known as respiratory frequency (f), as clarified next from the following equation:

The increase in Ve, which must occur when an increase in metabolic rate demands more oxygen, can be brought about through an increase in VT, f, or both.

Air flows into the alveoli through the nares, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. These structures constitute the conducting airways. Because gas exchange does not occur in these pathways, they are also known as the anatomic dead-space (Figure 45-3). Dead-space can also occur within the alveoli. This alveolar dead-space is caused by alveoli that are poorly perfused with blood, so that gas exchange cannot occur optimally (see Chapter 47). Physiologic dead-space is the sum of the anatomic and the alveolar dead-space. Let us define the portion of each VT enters the alveoli as Va and the part that enters the dead-space as Vd. Then:

If each side of this equation is multiplied by respiratory frequency (f) as follows:

The result is:

Therefore, minute ventilation (Ve) is the sum of alveolar ventilation (Va), which is essential for gas exchange, and dead­space ventilation (Vd), which is wasted ventilation.

Alveolar ventilation is regulated by control mechanisms to match the O₂ uptake and CO₂ elimination necessitated by metabolism. Thus, when an animal exercises, its alveolar ventilation increases to take in more O₁ and eliminate more CO₂.

FIGURE 45-3 Types of dead-space. The volume of the trachea and bronchi constitutes the anatomic dead-space; equipment dead-space is created by an endotracheal tube; and alveolar dead-space is the volume of air ventilating poorly perfused alveoli. Top, An unperfused alveolus is shown; bottom, a normally perfused alveolus; middle, an alveolus with less-than-optimal perfusion for the amount of ventilation received.

The fraction of each breath ventilating the dead-space is known as the dead-sρace∕tidal volume ratio (VD∕VT). The VD/VT varies considerably among species. In smaller species, such as dogs, it approximates 33%, whereas in some larger species, such as cattle and horses, it approximates 50% to 75%.

Because the volume of the anatomic dead-space is rela­tively constant, changes in VT, f, or both can alter the relative amounts of air that ventilate the alveoli and the dead-space. These changes in VT and f occur in animals during exercise and thermoregulation. For example, the small VT and high f char­acteristic of panting in dogs cause more air to ventilate the dead-space in order to cause water evaporation and heat loss. Cattle, pigs, and mules subjected to heat stress also increase respiratory rate and dead-space ventilation when trying to lose heat. In contrast to the effects of heat stress, cold-stressed animals have a higher metabolic rate, which is necessary to maintain body temperature in cold conditions. This leads to an increase in both O₂ consumption and CO₂ production, making it necessary for the animal to increase alveolar ventilation and decrease dead-space ventilation. Reducing the f and increasing VT accomplish the latter adaptations.

The veterinarian needs to ensure that equipment used for anesthesia or respiratory therapy does not increase the dead­space. Excessively long endotracheal tubes or overly large face masks create a large amount of equipment dead-space. The consequence of this is that the animal must take in a large VT to obtain adequate alveolar ventilation.